Low power radar level gauge system with integrated microwave circuit

ABSTRACT

The present invention relates to a radar level gauge system comprising a signal propagation device; a microwave signal source; a microwave signal source controller; a mixer configured to combine a transmit signal from the microwave signal source and a reflection signal from the surface to form an intermediate frequency signal; and processing circuitry coupled to the mixer and configured to determine the filling level based on the intermediate frequency signal. The microwave signal source is configured to exhibit a phase noise greater than or equal to −70 dBc/Hz @ 100 kHz offset from a carrier frequency for the transmit signal.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a radar level gauge system, and to amethod of determining a filling level of a product in a tank.

TECHNICAL BACKGROUND

Since radar level gauging was commercialized in the 1970's and 1980's,frequency modulated continuous wave (FMCW) gauging has been thedominating measuring principle for high accuracy applications. AnFMCW-type filling level measurement comprises transmitting into the tanka signal which is swept over a frequency range in the order of a fewGHz. For example, the signal can be in the range 24-27 GHz, or 9-10.5GHz. The transmitted signal is reflected by the surface of the productin the tank (or by any other impedance transition) and an echo signal,which has been delayed a certain time, is returned to the gauge. Theecho signal is combined with the transmitted signal in a mixer togenerate a combined signal, having a frequency equal to the frequencychange of the transmitted signal that has taken place during the timedelay. If a linear sweep is used, this frequency, which is also referredto as an intermediate frequency (IF), is proportional to the distance tothe reflecting surface. The combined signal from the mixer is oftenreferred to as an intermediate frequency signal or an IF-signal.

More recently, the FMCW principle has been improved, and today typicallyinvolves transmitting not a continuous frequency sweep but a signal withstepped frequency and practically constant amplitude—a stepped frequencysweep. When the transmitted and received signals are mixed, eachfrequency step will provide one constant piece of a piecewise constantIF-signal. The piecewise constant IF-signal is sampled and the sampledsignal is transformed to the frequency plane, for example using FFT, inorder to identify frequency components of the IF-signal. The frequencycomponents may then be translated to distances, for example in the formof an echo curve or similar.

Although highly accurate, conventional FMCW systems—continuous as wellas stepped—are relatively power hungry, which makes them less suitablefor applications where the power (and/or energy) is limited. Examples ofsuch applications include field devices powered by a two-wire interface,such as a 4-20 mA loop, and wireless devices powered by an internalenergy source (e.g. a battery or a solar cell).

For applications with limited available energy and/or power, also thecost and size of a level measuring system are often crucial parameters.Such applications may, for example, be found in the process industry.

To allow such applications to benefit from the high performance ofFMCW-type radar level gauging, it would be desirable to provide for amore compact and cost-efficient FMCW-type radar level gauge system,which is also capable of operating on the limited available power/energywith a reasonable update frequency.

SUMMARY

In view of the above, a general object of the present invention is toprovide for an improved radar level gauge system enabling accuratefilling level determination for applications with severely limitedsupply of energy and/or power. It is a further object of embodiments ofthe present invention to achieve this at a lower cost than withcurrently available radar level gauge systems of the FMCW-type.

According to a first aspect of the present invention, it is thereforeprovided a loop-powered radar level gauge system for determining thefilling level of a product in a tank and for providing a measurementsignal indicative of the filling level to a remote device via a two-wire4-20 mA current loop the two-wire 4-20 mA current loop being the onlysource of external power for the loop-powered radar level gauge system,the radar level gauge system comprising: loop interface circuitry forproviding the measurement signal to the two-wire current loop and forproviding power from the two-wire current loop to the radar level gaugesystem; a signal propagation device arranged to propagate anelectromagnetic transmit signal towards a surface of the product and toreturn an electromagnetic reflection signal resulting from reflection ofthe electromagnetic transmit signal at the surface; a microwave signalsource coupled to the signal propagation device and controllable togenerate the electromagnetic transmit signal, the microwave signalsource being configured to exhibit a phase noise greater than or equalto −70 dBc/Hz @ 100 kHz offset from a carrier frequency for the transmitsignal; a microwave signal source controller coupled to the microwavesignal source and configured to control the microwave signal source togenerate the transmit signal in the form of a measurement sweepcomprising a time sequence of discrete and mutually different frequencysteps defining a bandwidth of the transmit signal; a mixer coupled tothe microwave signal source and to the signal propagation device, andconfigured to combine the transmit signal and the reflection signal toform an intermediate frequency signal; and processing circuitry coupledto the mixer and configured to determine the filling level based on theintermediate frequency signal, wherein at least the microwave signalsource and the mixer are comprised in an integrated microwave circuit.

According to a second aspect of the present invention, it is provided alocally powered radar level gauge system for determining the fillinglevel of a product in a tank and for providing a measurement signalindicative of the filling level to a remote device using wirelesscommunication, comprising: a signal propagation device arranged topropagate an electromagnetic transmit signal towards a surface of theproduct and to return an electromagnetic reflection signal resultingfrom reflection of the electromagnetic transmit signal at the surface; amicrowave signal source coupled to the signal propagation device andcontrollable to generate the electromagnetic transmit signal, themicrowave signal source being configured to exhibit a phase noisegreater than or equal to −70 dBc/Hz @ 100 kHz offset from a carrierfrequency for the transmit signal; a microwave signal source controllercoupled to the microwave signal source and configured to control themicrowave signal source to generate the transmit signal in the form of ameasurement sweep comprising a time sequence of discrete and mutuallydifferent frequency steps defining a bandwidth of the transmit signal; amixer coupled to the microwave signal source and to the signalpropagation device, and configured to combine the transmit signal andthe reflection signal to form an intermediate frequency signal; andprocessing circuitry coupled to the mixer and configured to determinethe filling level based on the intermediate frequency signal; a wirelesscommunication unit connected to the processing circuitry for retrievingthe filling level from the processing circuitry and wirelesslytransmitting the measurement signal to the remote device; and a localenergy store for supplying energy sufficient for operation of the radarlevel gauge system, wherein at least the microwave signal source and themixer are comprised in an integrated microwave circuit.

According to a third aspect of the present invention, it is provided aradar level gauge system for determining the filling level of a productin a tank, comprising: a signal propagation device arranged to propagatean electromagnetic transmit signal towards a surface of the product andto return an electromagnetic reflection signal resulting from reflectionof the electromagnetic transmit signal at the surface; a microwavesignal source coupled to the signal propagation device and controllableto generate the electromagnetic transmit signal, the microwave signalsource being configured to exhibit a phase noise a phase noise greaterthan or equal to −70 dBc/Hz @ 100 kHz offset from a carrier frequencyfor the transmit signal; a microwave signal source controller coupled tothe microwave signal source and configured to control the microwavesignal source to generate the transmit signal in the form of ameasurement sweep comprising a time sequence of discrete and mutuallydifferent frequency steps defining a bandwidth of the transmit signal; amixer coupled to the microwave signal source and to the signalpropagation device, and configured to combine the transmit signal andthe reflection signal to form an intermediate frequency signal; andprocessing circuitry coupled to the mixer and configured to determinethe filling level based on the intermediate frequency signal, wherein atleast the microwave signal source and the mixer are comprised in anintegrated microwave circuit.

According to a fourth aspect of the present invention, it is provided amethod of determining a filling level of a product in a tank using aradar level gauge system comprising: a signal propagation device; amicrowave signal source for generating an electromagnetic transmitsignal coupled to the signal propagation device, the microwave signalsource comprises a voltage controlled oscillator; a mixer coupled to themicrowave signal source and to the signal propagation device, wherein atleast the microwave signal source and the mixer are comprised in anintegrated microwave circuit, wherein the method comprises the steps of:controlling the voltage controlled oscillator to such an operating pointthat a phase noise of the voltage controlled oscillator is greater thanor equal to −70 dBc/Hz @ 100 kHz offset from a carrier frequency for thetransmit signal; controlling the microwave signal source to generate thetransmit signal in the form of a measurement sweep comprising a timesequence of discrete and mutually different frequency steps defining abandwidth of the transmit signal; propagating, using the signalpropagation device, the transmit signal towards a surface of theproduct; returning, using the signal propagation device, a reflectionsignal resulting from reflection of the transmit signal at the surface;combining, using the mixer, the transmit signal and the reflectionsignal to form an intermediate frequency signal; and determining thefilling level based on the intermediate frequency signal.

An integrated microwave circuit should, in the context of the presentapplication, be understood to mean a type of monolithic (single die)integrated circuit (IC) device that operates at microwave frequencies(such as about 300 MHz to about 300 GHz).

An integrated microwave circuit is often referred to as an MMIC(Monolithic Microwave Integrated Circuit).

MMICs may, for example, be fabricated using SiGe, or a III-V compoundsemiconductor such as GaAs or InP.

The integrated microwave circuit may advantageously be comprised in amulti chip module together with one or several other integrated circuitsto provide more functionality to a single electronic component (definedby a single electronic component package).

The mixer may be provided in the form of any circuitry capable ofcombining the transmit signal and the reflection signal in such a waythat an intermediate frequency signal is formed that is indicative ofthe phase difference between the transmit signal and the reflectionsignal.

One example of a simple and compact mixer is the so-called single diodeleaky mixer.

In various embodiments, the electromagnetic transmit signal may havesubstantially constant amplitude. The power of the transmit signal maybe in the range of −50 dBm to +5 dBm, typically 0 dBm, i e 1 mW.

The present invention is based on a number of realizations.

Firstly, it has been realized that a key factor to reduce the cost of acurrently available FMCW-type radar level gauge system is to reduce thesize of the mechanical parts thereof, since these are often precisionmanufactured from high quality materials, such as high-grade stainlesssteel. Another important factor is the production yield, in particularof the microwave signal source (and the mixer).

Secondly, the present inventors have realized that both a reduction insize and an increase in the production yield can be achieved byproviding at least the microwave signal source and the mixer in the formof a single integrated microwave circuit (MMIC). Hereby, a large numberof discrete components can be replaced by a single component or a fewcomponents.

However, currently available integrated microwave circuits comprising amicrowave signal source are often intended for communicationapplications where the phase noise should be very low or for automotiveapplications, where the supply of energy/power is not an issue. It wouldtherefore appear that integrated microwave circuits are not suited forthe desired applications, where energy/power is scarce.

Because of the relatively short distance involved in radar level gaugingfor tanks, the requirements on phase noise is not as strict as for otherradar applications for greater ranges. Furthermore, a reduction in therequirement on phase noise allows the microwave signal source to bedesigned to consume less power. Accordingly, configuring the microwavesignal source to exhibit a phase noise greater than or equal to −70dBc/Hz @ 100 kHz offset from a carrier frequency for the transmit signalallows for reduced power consumption, while still providing forsufficient measurement performance for the measurement range of theradar level gauge system.

Hence, the present inventors have surprisingly found that, using anintegrated microwave circuit comprising at least the microwave signalsource and the mixer, wherein the microwave signal source is configuredto exhibit a phase noise greater than or equal to −70 dBc/Hz @ 100 kHzoffset from a carrier frequency for the transmit signal, allows asignificant reduction in the energy/power consumption of the radar levelgauge system while maintaining sufficient performance, for example withrespect to update rate. For instance, a filling level measurement may beperformed one time per second.

Tests and theoretical calculations show that a sweep duration of about10 ms should be sufficiently short to fulfill the energy/powerconsumption criteria for a two-wire current loop system, which iscurrently seen as the most challenging application.

Accordingly, all of the above-mentioned inventive insights contributesynergistically to provide for high accuracy FMCW-type radar levelgauging using a compact and cost-efficient radar level gauge system,that can still be powered using a two-wire communication interface or alocal energy source.

Advantageously, the microwave circuitry and measurement electronicscomponents may be mounted on the same circuit board, which facilitatesproduction and reduces cost.

In various embodiments of the radar level gauge system, the microwavesignal source controller may be configured to control the microwavesignal source to generate the measurement sweep having a time durationof less than 5 ms.

This may provide for even lower energy consumption because of a shorteron-time of the microwave signal source.

Moreover, the radar level gauge system according to various embodimentsof the present invention may further comprise a sampler coupled to themixer and configured to sample the intermediate frequency signal at lessthan 500 sampling times during the measurement sweep.

This provides for a further reduction in the energy consumption of theradar level gauge system, because the time used for processing theintermediate frequency signal can be reduced.

According to various embodiments, furthermore, the microwave signalsource controller and the sampler may be controllable independently ofeach other, in such a way that a duration of each of the frequency stepscomprised in the measurement sweep can be made different from a samplingtime interval between consecutive ones of the sampling times.

Hereby, the formation of the measurement sweep and the sampling intervalcan be independently controlled to tailor the operation of the radarlevel gauge system for an optimal combination of measurement performanceand power/energy consumption of different applications.

For example, the microwave signal source controller and the sampler mayadvantageously be controlled in such a way that the duration of each ofthe frequency steps of the measurement sweep is substantially shorterthan the sampling time interval.

This may reduce the risk of distortion of the intermediate frequencysignal due to large frequency steps, which improves the reliabilityand/or accuracy of measurement, in particular for long measurementdistances.

According to various embodiments of the present invention, the bandwidthof the transmit signal may be at least 1 GHz, whereby a sufficientresolution can be achieved for most applications.

In various embodiments of the radar level gauge system according to thepresent invention, the measurement sweep may be a frequency sweep from afirst frequency being the highest frequency of the frequency sweep to asecond frequency being the lowest frequency of the frequency sweep.

This enables improved operation of the radar level gauge system inembodiments where energy is stored in an energy store betweenmeasurement operations. Typically, the microwave signal source needs ahigher input voltage to provide a high frequency than to provide a lowfrequency—this is particularly the case for a microwave signal sourcecomprising a so-called voltage controlled oscillator (VCO). If energystorage, for example using one or several capacitor(s) is utilized, thecapability to provide a sufficiently high voltage for the highestfrequency of the frequency sweep will be greater in the beginning of themeasurement operation than at the end of the measurement operation.

According to various embodiments, furthermore, the microwave signalsource controller may advantageously comprise PLL circuitry and acrystal oscillator coupled to the PLL circuitry.

The PLL (phase lock loop or phase-locked loop) circuitry may, forexample, be a so-called analog or linear PLL (LPLL), a digital PLL(DPLL), an all digital PLL (ADPLL) or a software PLL (SPLL).

The PLL circuitry may advantageously be comprised in the same electroniccomponent package as the integrated microwave circuit, and the crystaloscillator may be arranged outside the electronic component package.

In various embodiments, the microwave signal source controller mayfurther comprise a low pass filter connected between the PLL and themicrowave signal source.

The low pass filter may advantageously be arranged outside theelectronic component package enclosing the integrated microwave circuit.

According to various embodiments of the present invention, the radarlevel gauge system may be controllable between at least a first sweepmode and a second sweep mode, wherein: in the first sweep mode, themicrowave signal source controller is configured to control themicrowave signal source to generate a first measurement sweep having afirst time duration and a first bandwidth; and in the second sweep mode,the microwave signal source controller is configured to control themicrowave signal source to generate a second measurement sweep having asecond time duration and a second bandwidth, at least one of the secondtime duration and the second bandwidth being substantially differentfrom the first time duration and the first bandwidth, respectively.

For any sampled FMCW system (continuous sweep or stepped), the maximummeasuring distance (range), L, is determined as:L=Nc/4B,

where N is the number of samples, c is the speed of light, and B is thebandwidth of the measurement sweep.

An increased bandwidth B gives an improved resolution, but from theabove relation it is clear that an increased bandwidth B will lead to areduced range L, unless the number of samples N is increased. However,as the sampling frequency is fixed at a reasonable value from an A/Dconversion standpoint, any increase of the number of samples willinevitably lead to an increased sweep time.

For a given measurement range, there is thus a tradeoff betweenresolution (bandwidth) on the one hand, and power consumption (sweeptime) on the other.

By providing a radar level gauge system that is controllable betweendifferent sweep modes as outlined above, different tradeoffs can, forinstance, be made for different intended measurement ranges. Inapplications where available power is very scarce, such as theloop-powered or battery-powered radar level gauge systems mentioned inthe Background section, the time durations of the measurement sweep maybe substantially the same in the different modes, and the bandwidth maybe tuned to allow for an increased measurement range at the expense ofmeasurement resolution. This extends the number of applications in whichembodiments of the radar level gauge system can be utilized.

According to various embodiments, the radar level gauge system of thepresent invention may advantageously be controllable between an activestate in which the microwave signal source is controlled to generate thetransmit signal, and an idle state in which no transmit signal isgenerated.

The radar level gauge system may further comprise an energy storeconfigured to store energy when the radar level gauge system is in theidle state and provide energy to the microwave signal source when theradar level gauge system is in the active state.

The local energy store may, for example, comprise a battery, acapacitor, and/or a super capacitor.

Moreover, the radar level gauge system may further comprise wirelesscommunication circuitry, such as a radio transceiver, for wirelesscommunication with a remote system.

It should be noted that the signal propagation device may be anysuitable radiating antenna or transmission line probe. Examples ofantennas include a horn antenna, a rod antenna, an array antenna and aparabolic antenna, etc. Examples of transmission line probes include asingle line probe (Goubau probe), a twin line probe and a coaxial probeetc.

It should also be noted that the processing circuitry may be provided asone device or several devices working together.

In summary, the present invention thus relates to a radar level gaugesystem comprising a signal propagation device; a microwave signalsource; a microwave signal source controller; a mixer configured tocombine a transmit signal from the microwave signal source and areflection signal from the surface to form an intermediate frequencysignal; and processing circuitry coupled to the mixer and configured todetermine the filling level based on the intermediate frequency signal.The microwave signal source is configured to exhibit a phase noisegreater than or equal to −70 dBc/Hz @ 100 kHz offset from a carrierfrequency for the transmit signal.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing acurrently preferred embodiment of the invention, wherein:

FIG. 1a schematically shows a process monitoring system comprising aradar level gauge system according to a first example embodiment of thepresent invention;

FIG. 1b schematically shows a process monitoring system comprising aradar level gauge system according to a second example embodiment of thepresent invention;

FIG. 2a is a block diagram schematically illustrating the radar levelgauge system in FIG. 1 a;

FIG. 2b is a block diagram schematically illustrating the radar levelgauge system in FIG. 1 b;

FIG. 3 is a schematic cross-section view of the radar level gauge systemin FIGS. 1a -b;

FIG. 4a is a schematic plane view of the measurement module comprised inthe radar level gauge system in FIG. 3,

FIG. 4b schematically shows the transceiver MCM (multi-chip module)comprised in the measurement module in FIG. 4 a;

FIG. 5 is a schematic block diagram of the radar level gauge system inFIG. 3;

FIG. 6 is a diagram schematically illustrating the available voltagelevel during a measurement sweep in an example embodiment;

FIG. 7 is a diagram schematically illustrating a measurement sweepaccording to an example embodiment of the present invention; and

FIG. 8 is a diagram schematically illustrating the intermediatefrequency signal that is sampled in order to determine the fillinglevel.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

In the present detailed description, various embodiments of the radarlevel gauge system according to the present invention are mainlydiscussed with reference to a radar level gauge system comprised in aprocess monitoring system and connected to a remote host by means of atwo-wire 4-20 mA communication loop which is also used for providingpower to the radar level gauge system, and to a battery-powered radarlevel gauge system with wireless communication capabilities.

It should be noted that this by no means limits the scope of the presentinvention, which equally well includes, for example, radar level gaugesystems that are not included in a process management system or radarlevel gauge systems that are not loop-powered or battery-powered.

FIG. 1a schematically shows a process monitoring system 1 comprising aplurality of field devices, including a first embodiment of a radarlevel gauge system 2 and a temperature sensing device 3 connected to ahost system 4 through a communication line 5 in the form of a 4-20 mAcurrent loop. Further field devices connected to the communication line5 are schematically indicated as boxes.

The radar level gauge system 2 and the temperature sensor 3 are botharranged on a tank containing a product 8 to be gauged.

In addition to providing signals on the current loop, typically in theform of a current value being indicative of a measurement value, thefield devices may be powered using the current provided by the currentloop 5. As has been previously discussed further above in the Backgroundand Summary sections, this severely limits the power that is availablefor operation of the field devices, in particular for active fielddevices, such as the radar level gauge system 2 in FIG. 1 a.

Following voltage conversion to the operating voltage(s) of the radarlevel gauge system 2, less than 30 mW may be available for the operationof the radar level gauge system.

As a consequence, FMCW-type radar level gauge systems have so far notbeen widely used in loop-powered applications, but so-called pulsedradar level gauge systems have instead been deployed. Even for suchlow-power pulsed radar level gauge systems, measures have been taken tomake more power available when needed in loop-powered applications. Forinstance, at least parts of the radar level gauge system have beenoperated intermittently and energy has been stored during inactive oridle periods to be used during active periods.

Solutions for intermittent operation and energy storage are, forexample, described in U.S. Pat. No. 7,952,514, U.S. Pat. No. 8,477,064and U.S. Ser. No. 12/603,048, each of which is hereby incorporated byreference in its entirety.

FIG. 1b schematically shows a process monitoring system 1 comprising aplurality of field devices, including a second embodiment of a radarlevel gauge system 2 and a temperature sensing device 3 that arewirelessly connected to the host system 4. In this second embodiment,the radar level gauge system is powered by a local energy store, such asa battery with a capacity greater than 0.5 Ah, and comprises acommunication antenna 9 to allow wireless communication with the hostsystem 4.

FIG. 2a is a block diagram schematically illustrating an exemplaryembodiment of the loop-powered radar level gauge system 2 in FIG. 1 a.

The radar level gauge system 2 in FIG. 2a comprises a measurement module12 for determining the filling level, and loop interface circuitry 112for providing a measurement signal S_(L) indicative of the filling levelto the two-wire current loop 5, and for providing power from thetwo-wire current loop 5 to the measurement module 12.

The loop interface circuitry 112 comprises current control circuitry inthe form of a controllable current source 114, a first DC-DC converter115 and voltage regulation circuitry 116.

During operation of the radar level gauge system 2, the controllablecurrent source 114 is controlled by the measurement module 12 to providethe measurement signal S_(L) to the two-wire current loop 5. Themeasurement signal S_(L) may be in the form of a the loop current I_(L)(a DC current level) and/or a an AC signal superimposed on the loopcurrent I_(L). An example of the latter case could be communication on a4-20 mA current loop according to the HART-protocol.

In the exemplary case that is schematically illustrated in FIG. 2a , itis assumed that the measurement signal S_(L) is provided in the form ofa certain loop current I_(L) between 4 mA and 20 mA.

The first DC-DC converter 115 has input terminals 118 a-b and outputterminals 119 a-b, where the input terminals 118 a-b are connected tothe two-wire current loop 5 in series with the controllable currentsource 114, and the output terminals are connected to the measurementmodule 12 to provide power from the two-wire current loop 5 to themeasurement module 12. The power from the two-wire 4-20 mA current loop5 is the only external power that is provided to the radar level gaugesystem 2.

The voltage regulation circuitry 116 monitors the voltage V_(cs) acrossthe current source 114 and controls the input voltage V_(in) of thefirst DC-DC converter to keep the voltage V_(cs) across the currentsource 114 substantially constant at a predetermined value, such as 2 V,when the loop voltage V_(L) varies. This may be realized in various waysby one of ordinary skill in the art. For example, the first converter115 may be a switching converter of the so-called “buck/boost” type.Such a converter may, for example, be realized in the form of aso-called SEPIC converter, which is well known to electrical engineers.The input voltage of a SEPIC converter can be controlled by controllinga switching transistor in the converter, for example using pulse widthmodulation.

However, practically any switching converter may be used in the fielddevice according to various embodiments of the present invention. Forexample, a forward converter or a flyback converter may be used.

On the output side of the converter 115, additional circuitry 121 may beprovided, which may have different configurations depending on thedesired function. Some examples of such additional circuitry 121 aredescribed in detail in U.S. Pat. No. 8,477,064, which is herebyincorporated by reference in its entirety.

When a new measurement signal S_(L) should be provided to the two-wirecurrent loop 5, the controllable current source 114 is controlled by themeasurement module 12 to provide a new loop current I_(L) to thetwo-wire current loop. In order to modify the loop current I_(L), thevoltage V_(cs) across the controllable current source should temporarilybe allowed to be changed. However, the voltage regulation circuitrystrives to keep the voltage V_(cs) across the controllable currentsource 114 constant. To allow for rapid and accurate changes in themeasurement signal, while at the same time keeping the voltage V_(cs)across the controllable current source substantially constant over time,the control of the controllable current source 114 may preferably befaster (have a shorter time constant) than the control of the voltageV_(in) across the input terminals 118 a-b of the first converter 115.

With reference to FIG. 2b , the second embodiment of the radar levelgauge system 2 in FIG. 1b comprises a measurement unit (MU) 210, awireless communication unit (WCU) 211 and a local energy store in theform of a battery 212. The wireless communication unit 211 mayadvantageously be compliant with WirelessHART (IEC 62591).

As is schematically indicated in FIG. 2b , the measurement unit 210comprises a first output 214, a second output 215, and a first input216. The first output 214 is connected to a first input 217 of thewireless communication unit 211 through a first dedicated discreet line,the second output 215 is connected to a second input 218 of the wirelesscommunication unit 211, and the first input 216 is connected to a firstoutput 219 of the wireless communication unit 211 through a seconddedicated discreet line. The second output 215 of the measurement unit210 and the second input 218 of the wireless communication unit 211 maybe configured to handle bidirectional data communication according to aserial or a parallel communication protocol to allow exchange of databetween the measurement unit 210 and the wireless communication unit211. The communication between the measurement unit 210 and the wirelesscommunication unit 211 using the different inputs/outputs is describedin more detail in U.S. patent application Ser. No. 13/537,513, which ishereby incorporated by reference in its entirety.

The above examples of a 4-20 mA current loop configuration and awireless and locally powered configuration are intended to give theskilled person detailed examples of how various aspects and embodimentsof the radar level gauge system according to the present invention canbe implemented. It should, however, be noted that there are many otherways of interfacing a radar level gauge system with a 4-20 mA currentloop and many other ways of configuring and controlling a wireless radarlevel gauge system that is powered by a local energy store. Such otherways are widely accessible to one of ordinary skill in the art and canbe implemented without excessive experimentation or undue burden.

Referring now to FIG. 3, which is a schematic and simplified explodedview of the radar level gauge system 2 in FIGS. 1a-b , the radar levelgauge system 2 comprises, an upper housing part 11, a measurement module12, a lower housing part 13, and a dielectric plug 14.

The lower housing part 13 comprises a waveguide portion and a conicalantenna portion (not shown), and the dielectric plug is formed to fillup and seal the opening of the conical antenna portion and the waveguideportion.

The measurement module 12, which will be described in further detailbelow with reference to FIGS. 4a-b , has a microwave electronics side 19and a measurement electronics side 20. On the microwave electronics side19, the measurement module 12 comprises components for generating,transmitting and receiving electromagnetic measurement signals in themicrowave frequency range, and a connector 22 for providing thetransmitted signals to the wave guide of the lower housing part 13.Various components are schematically indicated as simple boxes in FIG.2.

Referring now to FIG. 4a , which is a schematic plane view of themicrowave electronics side 19 of the measurement module 12, themicrowave electronics part—the transceiver—of the measurement module 12comprises a crystal oscillator 25, components forming a low pass filter26, and a chip radar component 27.

As is evident from FIG. 4a , the microwave electronics part of themeasurement module 12 is very compact and is formed by very fewcomponents. This is a key factor for being able to design a very compactFMCW-type radar level gauge system 2. In particular, referring againbriefly to FIG. 3, the upper housing part 11 and the lower housing part13 can be made using considerably less material than was previouslypossible, resulting in a cheaper, more compact radar level gauge system.

Furthermore, providing most of the functionality of the transceiver inthe chip radar component 27 improves the production yield of themeasurement module 12 and practically removes the need fortime-consuming component trimming and testing in production.

FIG. 4b schematically illustrates the chip radar component 27 that ismounted on the microwave electronics side 19 of the measurement module12. As is schematically indicated in FIG. 4b , the chip radar component27 is a QFP (quad flat pack) type component comprising a packagesubstrate 30, a first IC 31, a second IC 32, an encapsulating material33 and a plurality of pins 34 for connection of the chip radar component27 to the measurement module 12.

The first IC 31 is an integrated PLL component which is wire-bonded tothe package substrate 30. The integrated PLL component may, for example,be HMC 703 from Hittite or ADF 4158 from Analog Devices. The second IC32, which is also wire-bonded to the package substrate 30, is a custommade application specific integrated microwave circuit comprising themicrowave signal source and the mixer of the radar level gauge system 2.This integrated microwave circuit 32, which will be described in moredetail below with reference to FIG. 4, has been designed to exhibit aphase noise in the range −70 dBc/Hz to −50 dBc/Hz @ 100 kHz offset froma carrier frequency for the microwave signal generated and transmittedby the integrated microwave circuit 32.

FIG. 5 is a schematic block diagram of the radar level gauge system 2 inFIG. 2. As previously described, the radar level gauge system 2comprises microwave electronics 40 (on the microwave electronics side 19of the measurement module 12), measurement electronics 41 (on themeasurement electronics side 20 of the measurement module 12), andcommunication and power supply circuitry 42.

Referring to FIG. 5, the microwave electronics 40 comprises VCO (voltagecontrolled oscillator) 45, current supply circuitry 46, PLL 31, crystaloscillator 25, low-pass filter 26, and mixer 48.

The VCO 45, the current supply circuitry 46, and the mixer 48 arecomprised in the integrated microwave circuit (MMIC) 32 (referring alsoto FIG. 3b ). The PLL 31 and the MMIC 32 are included in the chip radarcomponent 27, and the crystal oscillator 25 and the low pass filter 26are provided as discrete components outside the chip radar component 27.

The measurement electronics 41 comprises sampler 51, ND-converter 52,and microprocessor 55.

The communication and power supply circuitry 42 comprises a power supplymodule 57 and a communication interface module 58.

In operation, the VCO 45 is controlled by a microwave signal sourcecontroller comprising the crystal oscillator 25, the PLL 31 and thelow-pass filter to generate an electromagnetic transmit signal.

The current supply circuitry 46 is configured to bias the VCO 45 at anoperating point at which the VCO 45 exhibits a phase noise in the range−70 dBc/Hz to −50 dBc/Hz @ 100 kHz offset from a carrier frequency forthe transmit signal. This will allow more energy efficient operation ofthe VCO than in existing chip radar components at the expense of ahigher phase noise. As has been previously mentioned, however, in theparticular application of level gauging in tanks, sufficient (and high)sensitivity can be achieved even at a relatively high phase noise leveldue to the relatively short measurement distance.

As is schematically indicated in FIG. 5, the transmit signal TX isprovided to the signal propagation circuitry (antenna or transmissionline probe) which propagates the transmit signal TX towards the surfaceof the product 8 in the tank 7. The transmit signal TX is reflected atthe surface, and a reflection signal RX is returned to the microwaveelectronics 40 of the radar level gauge system 2. In particular, thereflection signal RX is provided to the mixer 48, where the reflectionsignal RX is combined with the transmit signal TX to form anintermediate frequency signal IF.

The intermediate frequency signal IF is routed from the microwaveelectronics 40 to the measurement electronics 41, where the signal IF issampled by sampler 51 and the sampled signal values are converted todigital form by the A/D-converter 52 before being provided to themicroprocessor 55, where the filling level is determined. In addition todetermining the filling level, the microprocessor 55 controls the PLL 31and communicates with a remote device via the communication interfacemodule 58.

In the radar level gauge system 2 of FIG. 5, the PLL 31 and the sampler51 (and the A/D-converter 52) are independently controllable, so thatthe microprocessor 55 can control the PLL 31 to in turn control the VCO45 to generate frequency steps with a certain time duration t_(step),and at the same time control sampling to take place with a samplinginterval t_(sample) that is different from the time duration t_(step) ofthe frequency steps.

Moreover, the microprocessor may store, internally or externally,parameters corresponding to different sweep modes. Such different sweepmodes may, for example, be adapted for different measurement ranges L.The microprocessor 55 may receive a command via the communicationinterface 58 to switch to a different sweep mode. In response, themicroprocessor 55 may then access the stored parameters related to therequested sweep mode, and control at least the PLL 31 in accordance withthe new parameters.

Examples of sweep mode parameters are provided in the table below:

Measurement range (L) Bandwidth (B) Sweep time (t_(sweep)) 20 meters 2GHz 4-5 ms 40 meters 1 GHz 4-5 ms

The microwave electronics 40 and the measurement electronics 41 arepowered via the power supply module 57, which may advantageouslycomprise an energy store, such as one or several capacitor(s) forstoring energy when energy is available on the current loop 5 andproviding energy to the microwave electronics 40 and/or the measurementelectronics 41 when more energy is required than is available on thecurrent loop 5.

Due to the limited energy storage capability of a capacitor or similar,the supply voltage from the power supply module 57 to the microwaveelectronics may decrease as a result of current being drawn from thepower supply module 57. This effect is schematically indicated in FIG.5. During a measurement operation, the output voltage from the powersupply module 57 may, referring to FIG. 5, decrease from V_(start) toV_(end).

Depending on the amount of energy stored in the power supply module 57,the voltage V_(end) at the end of the measurement operation may not besufficient to control the VCO 45 to generate the highest frequency (f₁in FIG. 6) of the measurement frequency sweep.

Therefore, in embodiments of the present invention, the VCO 45 may becontrolled to generate the transmit signal TX as a frequency sweep froma high frequency f₁ to a low frequency f₂, instead of in theconventional manner from a low frequency to a high frequency. This willdecrease the risk of incorrect frequencies being generated due to adepleted energy store.

In FIG. 7, a measurement sweep is shown comprising a time sequence ofdiscrete and mutually different frequency steps, defining a bandwidth Bof the transmit signal TX. Referring to FIG. 7, the bandwidth B=f₂. Thetime duration t_(sweep) of the measurement sweep is less than 10 ms. Theduration of each frequency step is, as is indicated in FIG. 7, t_(step),and the frequency difference between adjacent (in terms of frequency)frequency steps is f_(step).

Due to the relatively short distance from the radar level gauge system 2to the surface of the product 8 in the tank 7 (compared to the speed oflight), the reflection signal RX will (almost) always have the samefrequency as the stepped transmit signal TX, but with (in this case)decreasing phase difference from the start of the measurement sweep tothe end of the measurement sweep.

This decreasing phase difference will correspond to the frequencydifference that would have been obtained using continuous FMCW, and theintermediate frequency signal IF will, in the time domain, look like thestepped sine wave schematically shown in FIG. 8.

In the exemplary case illustrated by the IF-signal in FIG. 8, the PLL 31has been controlled to, in turn, control the VCO 45 to generatefrequency steps with a step time t_(step), and the sampler 51 has beencontrolled to sample the IF-signal with a sampling interval t_(sample)between consecutive sampling times that is considerably longer than thestep time t_(step). This may be advantageous since the generation of thetransmit signal TX with a relatively short step time t_(step) does not“cost” much extra power while considerably reducing the risk of “falseechoes” due to distortion of the IF-signal. On the other hand, thesampling frequency can be kept down (the sampling interval t_(sample)kept longer) to reduce the power consumption of the measurementelectronics 41.

The person skilled in the art realizes that the present invention by nomeans is limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within thescope of the appended claims.

What is claimed is:
 1. A loop-powered radar level gauge system fordetermining the filling level of a product in a tank and for providing ameasurement signal indicative of said filling level to a remote devicevia a two-wire 4-20 mA current loop said two-wire 4-20 mA current loopbeing the only source of external power for said loop-powered radarlevel gauge system, said radar level gauge system comprising: loopinterface circuitry for providing said measurement signal to thetwo-wire current loop and for providing power from said two-wire currentloop to said radar level gauge system; a signal propagation devicearranged to propagate an electromagnetic transmit signal towards asurface of the product and to return an electromagnetic reflectionsignal resulting from reflection of the electromagnetic transmit signalat the surface; a microwave signal source coupled to said signalpropagation device and controllable to generate said electromagnetictransmit signal, said microwave signal source being configured toexhibit a phase noise greater than or equal to −70 dBc/Hz @ 100 kHzoffset from a carrier frequency for said transmit signal; a microwavesignal source controller coupled to said microwave signal source andconfigured to control said microwave signal source to generate saidtransmit signal in the form of a measurement sweep comprising a timesequence of discrete and mutually different frequency steps defining abandwidth of said transmit signal; a mixer coupled to said microwavesignal source and to said signal propagation device, and configured tocombine said transmit signal and said reflection signal to form anintermediate frequency signal; and processing circuitry coupled to saidmixer and configured to determine said filling level based on saidintermediate frequency signal, wherein at least said microwave signalsource and said mixer are comprised in an integrated microwave circuit.2. The radar level gauge system according to claim 1, wherein saidmicrowave signal source is configured to exhibit a phase noise smallerthan −50 dBc/Hz @ 100 kHz offset from a carrier frequency for saidtransmit signal.
 3. The radar level gauge system according to claim 1,wherein said microwave signal source comprises a voltage controlledoscillator.
 4. The radar level gauge system according to claim 3,further comprising current supply circuitry configured to maintain saidvoltage controlled oscillator at such an operating point that a phasenoise of said voltage controlled oscillator is in the range of −70dBc/Hz to −50 dBc/Hz @ 100 kHz offset from a carrier frequency for saidtransmit signal.
 5. The radar level gauge system according to claim 4,wherein said current supply circuitry is comprised in said integratedmicrowave circuit.
 6. The radar level gauge system according to claim 1,wherein said measurement sweep has a time duration of less than 10 ms.7. The radar level gauge system according to claim 6, wherein saidmeasurement sweep has a time duration of less than 5 ms.
 8. The radarlevel gauge system according to claim 1, wherein said bandwidth of thetransmit signal is at least 2.5 GHz.
 9. The radar level gauge systemaccording to claim 1, wherein said radar level gauge system isconfigured to provide a first transmit signal having a first carrierfrequency and a second transmit signal having a second carrier frequencybeing at least 1.5 times higher than said first transmit signal.
 10. Theradar level gauge system according to claim 1, wherein said microwavesignal source controller comprises PLL circuitry and a crystaloscillator coupled to the PLL circuitry.
 11. The radar level gaugesystem according to claim 1, wherein said radar level gauge system iscontrollable between at least a first sweep mode and a second sweepmode, wherein: in said first sweep mode, said microwave signal sourcecontroller is configured to control said microwave signal source togenerate a first measurement sweep having a first time duration and afirst bandwidth; and in said second sweep mode, said microwave signalsource controller is configured to control said microwave signal sourceto generate a second measurement sweep having a second time duration anda second bandwidth, at least one of said second time duration and saidsecond bandwidth being substantially different from said first timeduration and said first bandwidth, respectively.
 12. The radar levelgauge system according to claim 11, wherein said second bandwidth is atleast 1.5 times said first bandwidth.
 13. The radar level gauge systemaccording to claim 1, wherein said radar level gauge system iscontrollable between an active state in which said microwave signalsource is controlled to generate said transmit signal, and an idle statein which no transmit signal is generated.
 14. The radar level gaugesystem according to claim 13, further comprising an energy storeconfigured to store energy when the radar level gauge system is in saididle state and provide energy to said microwave signal source when theradar level gauge system is in said active state.
 15. A locally poweredradar level gauge system for determining the filling level of a productin a tank and for providing a measurement signal indicative of saidfilling level to a remote device using wireless communication,comprising: a signal propagation device arranged to propagate anelectromagnetic transmit signal towards a surface of the product and toreturn an electromagnetic reflection signal resulting from reflection ofthe electromagnetic transmit signal at the surface; a microwave signalsource coupled to said signal propagation device and controllable togenerate said electromagnetic transmit signal, said microwave signalsource being configured to exhibit a phase noise greater than or equalto −70 dBc/Hz @ 100 kHz offset from a carrier frequency for saidtransmit signal; a microwave signal source controller coupled to saidmicrowave signal source and configured to control said microwave signalsource to generate said transmit signal in the form of a measurementsweep comprising a time sequence of discrete and mutually differentfrequency steps defining a bandwidth of said transmit signal; a mixercoupled to said microwave signal source and to said signal propagationdevice, and configured to combine said transmit signal and saidreflection signal to form an intermediate frequency signal; andprocessing circuitry coupled to said mixer and configured to determinesaid filling level based on said intermediate frequency signal; awireless communication unit connected to said processing circuitry forretrieving said filling level from said processing circuitry andwirelessly transmitting said measurement signal to the remote device;and a local energy store for supplying energy sufficient for operationof said radar level gauge system, wherein at least said microwave signalsource and said mixer are comprised in an integrated microwave circuit.16. The radar level gauge system according to claim 15, wherein saidmicrowave signal source is configured to exhibit a phase noise smallerthan −50 dBc/Hz @ 100 kHz offset from a carrier frequency for saidtransmit signal.
 17. The radar level gauge system according to claim 15,wherein said microwave signal source comprises a voltage controlledoscillator.
 18. The radar level gauge system according to claim 17,further comprising current supply circuitry configured to maintain saidvoltage controlled oscillator at such an operating point that a phasenoise of said voltage controlled oscillator is in the range of −70dBc/Hz to −50 dBc/Hz @ 100 kHz offset from a carrier frequency for saidtransmit signal.
 19. The radar level gauge system according to claim 18,wherein said current supply circuitry is comprised in said integratedmicrowave circuit.
 20. The radar level gauge system according to claim15, wherein said local energy store is a battery having an energystorage capacity of at least 0.5 Wh.
 21. The radar level gauge systemaccording to claim 15, wherein said measurement sweep has a timeduration of less than 10 ms.
 22. The radar level gauge system accordingto claim 21, wherein said measurement sweep has a time duration of lessthan 5 ms.
 23. The radar level gauge system according to claim 15,wherein said bandwidth of the transmit signal is at least 2.5 GHz. 24.The radar level gauge system according to claim 15, wherein said radarlevel gauge system is configured to provide a first transmit signalhaving a first carrier frequency, such as around 6 GHz, and a secondtransmit signal having a second carrier frequency being at least 1.5times higher than said first transmit signal, such as around 24 GHz. 25.The radar level gauge system according to claim 15, wherein saidmicrowave signal source controller comprises PLL circuitry and a crystaloscillator coupled to the PLL circuitry.
 26. The radar level gaugesystem according to claim 15, wherein said radar level gauge system iscontrollable between at least a first sweep mode and a second sweepmode, wherein: in said first sweep mode, said microwave signal sourcecontroller is configured to control said microwave signal source togenerate a first measurement sweep having a first time duration and afirst bandwidth; and in said second sweep mode, said microwave signalsource controller is configured to control said microwave signal sourceto generate a second measurement sweep having a second time duration anda second bandwidth, at least one of said second time duration and saidsecond bandwidth being substantially different from said first timeduration and said first bandwidth, respectively.
 27. The radar levelgauge system according to claim 26, wherein said second bandwidth is atleast 1.5 times said first bandwidth.
 28. The radar level gauge systemaccording to claim 15, wherein said radar level gauge system iscontrollable between an active state in which said microwave signalsource is controlled to generate said transmit signal, and an idle statein which no transmit signal is generated.
 29. The radar level gaugesystem according to claim 28, further comprising an energy storeconfigured to store energy when the radar level gauge system is in saididle state and provide energy to said microwave signal source when theradar level gauge system is in said active state.
 30. A radar levelgauge system for determining the filling level of a product in a tank,comprising: a signal propagation device arranged to propagate anelectromagnetic transmit signal towards a surface of the product and toreturn an electromagnetic reflection signal resulting from reflection ofthe electromagnetic transmit signal at the surface; a microwave signalsource coupled to said signal propagation device and controllable togenerate said electromagnetic transmit signal, said microwave signalsource being configured to exhibit a phase noise greater than or equalto −70 dBc/Hz @ 100 kHz offset from a carrier frequency for saidtransmit signal; a microwave signal source controller coupled to saidmicrowave signal source and configured to control said microwave signalsource to generate said transmit signal in the form of a measurementsweep comprising a time sequence of discrete and mutually differentfrequency steps defining a bandwidth of said transmit signal; a mixercoupled to said microwave signal source and to said signal propagationdevice, and configured to combine said transmit signal and saidreflection signal to form an intermediate frequency signal; andprocessing circuitry coupled to said mixer and configured to determinesaid filling level based on said intermediate frequency signal, whereinat least said microwave signal source and said mixer are comprised in anintegrated microwave circuit.
 31. A method of determining a fillinglevel of a product in a tank using a radar level gauge systemcomprising: a signal propagation device; a microwave signal source forgenerating an electromagnetic transmit signal coupled to said signalpropagation device, said microwave signal source comprises a voltagecontrolled oscillator; a mixer coupled to said microwave signal sourceand to said signal propagation device, wherein at least said microwavesignal source and said mixer are comprised in an integrated microwavecircuit, wherein said method comprises the steps of: controlling saidvoltage controlled oscillator to such an operating point that a phasenoise of said voltage controlled oscillator is greater than or equal to−70 dBc/Hz @ 100 kHz offset from a carrier frequency for said transmitsignal; controlling said microwave signal source to generate saidtransmit signal in the form of a measurement sweep comprising a timesequence of discrete and mutually different frequency steps defining abandwidth of said transmit signal; propagating, using said signalpropagation device, said transmit signal towards a surface of theproduct; returning, using said signal propagation device, a reflectionsignal resulting from reflection of the transmit signal at the surface;combining, using said mixer, said transmit signal and said reflectionsignal to form an intermediate frequency signal; and determining saidfilling level based on said intermediate frequency signal.
 32. Themethod according to claim 31, wherein said microwave signal source iscontrolled to generate said measurement sweep for a time duration ofless than 10 ms.
 33. The method according to claim 31, wherein said stepof determining said filling level comprises the step of: sampling saidintermediate frequency signal at less than 500 sampling times duringsaid sweep.
 34. The method according to claim 31, wherein saidmeasurement sweep comprises a first number of discrete and mutuallydifferent frequency steps; and wherein said step of determining saidfilling level comprises the step of: sampling said intermediatefrequency signal at a second number of sampling times during saidmeasurement sweep, said first number being substantially greater thansaid second number.